1. Field of the Invention
The present invention relates to an on-vehicle radar system (i.e., radar system installed on a motor vehicle) which is implemented on the basis of an FM pulse Doppler radar. Further, the present invention also relates to an on-vehicle radar system whose operation mode can be changed over between a pulse operation mode and an FMCW (Frequency Modulation Continuous Wave) operation mode at an appropriate timing. More particularly, the present invention is concerned with an on-vehicle radar system which is imparted with a function for arithmetically determining a range correcting value with a high accuracy regardless of the width of range gate which may be set to an optional valve.
2. Description of Related Art
As a hitherto known or conventional on-vehicle radar system, there may be mentioned the one which is implemented on the basis of an FM (Frequency Modulation) pulse Doppler radar. For more particulars, reference may have to be made to e.g. Japanese Patent Application Laid-Open Publication No. 264426/2001 (JP-A-2001-264426).
The conventional on-vehicle radar system is comprised of a vehicle velocity sensor for determining the traveling speed of the motor vehicle equipped with the radar system (hereinafter referred to as the concerned motor vehicle only for the convenience of description), a modulation control voltage generator, a voltage-controlled oscillator for generating an electromagnetic wave signal having a transmission frequency on the order of 76 GHz to 77 GHz, a transmission/reception changeover switch for changing over the electromagnetic wave power supplied to the transmitting amplifier or the mixer for reception, a transmitting amplifier for amplifying the electromagnetic wave power to a transmitting amplifier or a mixer for reception, a transmitting antenna for radiating the amplified electromagnetic wave into space, a receiving antenna for receiving a reflection wave (echo signal) of the electromagnetic wave radiated and reflected on a target (object to be detected), a receiving amplifier for amplifying the received electromagnetic wave signal, a mixer for mixing the radiated electromagnetic wave signal and the reflected electromagnetic wave signal to thereby generate as the output thereof a beat signal indicative of a range (or distance) to the target and a relative velocity thereof, a low-pass filter for converting a cut-off frequency to the reciprocal of a pulse time duration (i.e., pulse width) of the transmitted electromagnetic wave signal, an AGC (Automatic Gain Control) amplifier for adjusting or controlling the gain in dependence on the received power of the reflection wave (echo signal), an A/D (Analog-to-Digital) converter for converting the beat signal to a digital signal, and a range arithmetic module (or range/relative velocity arithmetic module) for arithmetically determining a range to the target as well as the relative velocity thereof on the basis of the digital value of the beat signal resulting from the A/D conversion.
The range arithmetic module includes a range correcting module designed for correcting the range to the target arithmetically determined on the basis of the range to the target and the relative velocity thereof, the range gate and the velocity of the concerned motor vehicle.
Next, description will be made of the electromagnetic wave transmitting/receiving operation of the conventional on-vehicle radar system implemented in the structure described above.
The voltage-controlled oscillator is designed to output the electromagnetic wave signal modulated in conformance with a voltage signal supplied from the modulation control voltage generator. The modulated electromagnetic wave signal is fed to the transmitting amplifier by way of the transmission/reception changeover switch to be thereby amplified and radiated into space through the medium of the transmitting antenna.
In succession, at a time point corresponding to the time lapse of a pulse duration of e.g. 33.3 ns (= 1/30 MHz, which corresponds to the range of 5 m) from the time point at which radiation of the electromagnetic wave into space was started, the transmission/reception changeover switch is changed over to the receiving mode or state, as the result of which the voltage-controlled oscillator and the mixer are connected to each other.
The electromagnetic wave radiated into space from the transmitting antenna is in the form of pulses each having the duration (time width) of 33.3 ns. The electromagnetic pulse wave is reflected at a target distanced from the concerned motor vehicle for a certain range or distance to be received by the on-vehicle radar system. More specifically, the received electromagnetic pulse wave (echo signal) caught by the receiving antenna with a delay time which depends on the range to the target relative to the radiation or transmission of the electromagnetic wave.
In the case where the target is moving relative to the concerned motor vehicle, i.e., when the target is traveling at a relative velocity, the frequency of the received electromagnetic wave signal (echo signal) is inputted to the receiving antenna, being shifted from the frequency of the transmitted electromagnetic wave signal by a predetermined frequency corresponding to the relative velocity of the target under the influence of the Doppler effect.
The electromagnetic wave signal inputted through the receiving antenna is amplified by the receiving amplifier to be subsequently mixed with the transmitted electromagnetic wave signal supplied from the voltage-controlled oscillator by means of the mixer, whereby the beat signal is outputted from the mixer.
The beat signal thus acquired is then forced to pass through the filter having the cut-off frequency of e.g. 30 MHz, as a result of which a signal having a frequency component equivalent to a frequency difference between the frequency of the transmitted electromagnetic wave signal and that of the reflected wave signal (echo) is made available. This signal is referred to as the beat frequency signal. The beat frequency signal is then amplified by the AGC amplifier to be inputted to the A/D converter and converted to a digital signal.
In succession, in the range arithmetic module, the range to the target and the relative velocity thereof are arithmetically determined on the basis of the output data of the A/D converter (i.e., digital data of the beat signal resulting from the A/D conversion). This arithmetic processing procedure will be elucidated below.
For the simplification of elucidation, it is assumed that frequency modulation is not performed by the voltage-controlled oscillator and that the frequency of the transmission signal is 76.5 GHz.
In the case where a predetermined velocity resolution (=1 km/h) is to be acquired, the resolution Δf of the Doppler frequency is determined in accordance with the undermentioned expression (1):
                              Δ          ⁢                                          ⁢          f                =                                            2              ⁢              Δ              ⁢                                                          ⁢              v                        λ                    ⁢                                          ⁢                                          =                                                    2                ×                0.2777                ⁢                                                                  ⁢                m                ⁢                                  /s                                                            0.003921                ⁢                                                                  ⁢                m                                      ⁢                                                  ⁢                                                  =                                          141.64                ⁢                                                                  ⁢                                  (                  Hz                  )                                            ⁢                                                          ⁢                                                          =                                                1                                      7.05977                    ⁢                                                                                  ⁢                                          (                      ms                      )                                                                      =                                  1                                      T                    ⁢                                                                                  ⁢                    m                                                                                                          (        1        )            
As is obvious from the above expression (1), a measuring time Tm of 7.06 ms is required.
At this juncture, let's assume that the maximum measurable range is e.g. 150 m. Then, the transmission wave output period is 33.3 ns×30 (=1 μs). Accordingly, for realizing the velocity resolution of “1 km/h”, it is necessary to acquire the transmission wave outputs on a range-gate base (corresponding to 7060 outputs) and perform the FFT (Fast Fourier Transform) arithmetic on all the data of the beat signals in every range gate. Through this procedure, the Doppler shift in the range gate corresponding to the detection time point can be determined.
In this conjunction, the range Rg to the target and the relative velocity V thereof can arithmetically be determined in accordance with the undermentioned expressions (2) and (3), respectively.
                    Rg        =                              tg            ×            n            ×            C                    2                                    (        2        )                                V        =                              fb1            ×            C                                2            ×            f0                                              (        3        )            where tg represents the time width of the range gate (pulse time width),                n represents the ID (identifier) number of the range gate,        C represents the velocity of light,        fb1 represents the beat frequency, and        f0 represents the transmission frequency (=76.5 GHz).        
At this juncture, in consideration of the fact that the transmitted electromagnetic wave signal whose frequency is so modulated as to repetitively increase and decrease, it is assumed that during the measuring period Tm (=7.06 ms), the transmission frequency increases at a constant rate from 76.425 GHz to 76.575 GHz in the band width B (=150 MHz).
In that case, the round-trip time t taken for the electromagnetic wave signal radiated from the transmitting antenna to be caught by the receiving antenna after having been reflected at the target can be determined in accordance with the undermentioned expression (4):
                    t        =                              range            ×            2                    C                                    (        4        )            
Since the transmission frequency increases during the round-trip time period t, the beat frequency fbu is determined as a sum of the frequency difference fb2 between the transmission frequency and the reception frequency which difference depends on the distance and the Doppler frequency fb1 ascribable to the relative velocity of the target, as given by the following expression (5):fbu=fb2+fb1  (5)
Next, it is assumed that during the succeeding measuring period Tm (=7.06 ms), the transmitted signal frequency decreases or lowers at a predetermined constant rate from 76.425 GHz to 76.575 GHz in the band width B (=150 MHz).
In this case, since the transmission frequency decreases during the round-trip time t taken for the electromagnetic wave signal radiated from the transmitting antenna to be caught by the receiving antenna after reflection at the target, the beat frequency fbd is represented by a sum of the frequency difference fb2′ between the transmitted frequency and the received frequency which difference depends on the range and the Doppler frequency fb1′ ascribable to the relative velocity of the target.
Incidentally, the range and the relative velocity during the frequency decreasing (i.e., in the frequency decreasing pulse, to say in another way) may duly be regarded as being equal to those when the frequency is increasing (i.e., in the frequency increasing phase). Further, the constant frequency increasing rate is equal to the constant frequency decreasing rate. Consequently, it can duly be regarded that fb1=fb1′ and that fb2′=−fb2. Thus, the beat frequency fbd during the frequency decreasing or lowering can be given by the following expression (6):fbd=fb2′+fb1′=−fb2+fb1  (6)
As is obvious from the above, by increasing and decreasing the transmission frequency to thereby determine the beat frequencies fbu and fbd, respectively, the frequency difference fb2 between the transmission frequency and the reception frequency which difference depends on the range to the target and the Doppler frequency fb1 ascribable to the relative velocity thereof can be determined, respectively, in accordance with the following expressions (7):
                              fb1          =                                    fbu              +              fbd                        2                          ,                  fb2          =                                    fbu              -              fbd                        2                                              (        7        )            
In this conjunction, it is noted that since the frequency difference fb2 represents the frequency increase or frequency decrease during the round-trip time t determined in accordance with the expression (4), the relation given by the undermentioned expression (8) applies valid:
                              fb2          B                =                  t          Tm                                    (        8        )            
From the expressions (4) and (8), the distance or range Rb from the concerned motor vehicle to the target can be determined on the basis of the frequency difference fb2 in accordance with the following expression (9):
                    Rb        =                                            Tm              ×              C                                      2              ×              B                                ×          fb2                                    (        9        )            
Further, the relative velocity V of the target can be determined on the basis of the Doppler frequency fb1 in accordance with the expression (3) mentioned previously.
From the expression (9), it can be seen that the range Rb and the frequency difference fb2 bear a proportional relation to each other. Accordingly, the range resolution ΔR can be given by the following expression (10):
                              Δ          ⁢                                          ⁢          R                =                                                            Tm                ×                C                                            4                ×                B                                      ⁢            Δ            ⁢                                                  ⁢            f                    =                      C                          2              ×              B                                                          (        10        )            where Δf (=1/(Tm/2)) represents the frequency resolution of the frequency difference fb2 between the transmission frequency and the reception frequency.
In the expression (10), when the band width B=300 MHz, the range resolution ΔR is then “0.5 m”, which means that the range resolution is improved over that of the range Rg determined in accordance with the expression (2) mentioned previously.
Further, even when noise is generated for some cause, as a result of which beat frequency ascribable to the noise is detected in a given range gate, the noise component can be eliminated from the detected data so far as the error or difference between the range Rg determined in accordance with the expression (2) and the range Rb determined in accordance with the expression (9) is not smaller than the width (=5 m) of the range gate.
By way of example, let's assume that the range to the target object is 52 m and that the relative velocity of the target is “0” km/h, the range determined in accordance with the expression (2) is “50 m” while the range Rb determined in accordance with the expression (9) is “52 m”.
In this conjunction, it is again assumed that error occurs in the voltage applied to the voltage-controlled oscillator due to variance among the elements, temperature change and/or for other cause, the error being given by “voltage applied to the voltage-controlled oscillator”−“voltage of the oscillation frequency resulting from the conversion”, and that the band width B changes by a factor of “0.9” due to the error, the range Rg determined in accordance with the expression (2) is then “50 m”, whereas the range Rb determined in accordance with the expression (9) is “52/0.9 (≈58 m)”.
In the case where error makes appearance between the range Rg determined in accordance with the expression (2) and the range Rb determined in accordance with the expression (9), it is impossible to discern whether the range error is ascribable to the noise mentioned previously or the range error has actually taken place.
Under the circumstances, by taking it into consideration that when it is decided that the velocity of the concerned motor vehicle is “0” km/h (e.g. when the concerned motor vehicle is stopping in the engine operation starting state), then the relative velocities of stationary objects in the surroundings are “0” km/h and that many of the targets in the surroundings are stationary, the range correcting module is so designed as to select the target whose relative velocity is “0” km/h from the detected targets and make decision that error has been brought about in the band width B when the difference between the ranges Rg and Rb determined in accordance with the expressions (2) and (9), respectively, become greater than the range gate width (=5 m) inclusive.
As is apparent from the above, when the range to the target is 52 m with the relative velocity of the target being “0” km/h, the range Rb determined in accordance with the expression (2) is “58 m”. However, determining the range gate ID number n inversely from the range Rb, then n=11 from the expression (2).
However, the ID number n of the actually detected range gate is “10”. Accordingly, the correcting value k in this case can be determined as follows:
                    k        =                  10          11                                    (        11        )            
The range error can be reduced by correcting the range Rb determined in accordance with the expression (2) by using the above-mentioned correcting value k in accordance with the undermentioned expression (12):
                              Rf          ′                =                              Rf            ×            k                    =                                    58              ×                              10                11                                      ≈                          53              ⁢                                                          ⁢              m                                                          (        12        )            
Next, description will be directed to the processing procedure for arithmetically determining or computing the range correcting value in the conventional on-vehicle radar system described above.
The arithmetic processing routine for computing the range correcting value is called from a main control processing procedure periodically at a predetermined interval.
When the velocity of the concerned motor vehicle is greater than “0” km/h, the main control processing procedure is resumed without executing the arithmetic processing routine. On the other hand, when the velocity of the concerned motor vehicle is “0” km/h, the range gate of interest is initialized to a minimum range gate.
In succession, when the range gate condition is such that “range gate of interest”>“final range gate”, the arithmetic processing routine is terminated, whereas when “range gate of interest”≦“final range gate”, check flags for all the beat frequency components fbu[i] (i=0, 1, . . . ) in the frequency increasing phase within the range gate of interest are reset.
Subsequently, when all the beat frequency components fbu[i] in the frequency increasing phase have been checked, then the range gate of interest is incremented by “+1”, whereon the decision processing for the range gate condition mentioned previously is resumed.
On the other hand, unless all the beat frequency components fbu[i] in the frequency increasing phase have been checked, one unchecked component is selected from the beat frequency components fbu[i], whereon the check flag for the selected unchecked component is set to the state indicating “checked”.
Thereafter, the check flags for all the beat frequency components fbd[j] (j=0, 1, 2, . . . ) in the frequency decreasing phase within the range gate of interest are reset. When all the beat frequency components fbd[j] have been checked, the check/uncheck decision processing for the beat frequency components fbu[i] in the frequency increasing phase is resumed.
On the other hand, unless all the beat frequency components fbd[j] in the frequency decreasing phase have been checked, one unchecked component is selected from the beat frequency components fbd[j], whereon the check flag for the selected unchecked component is set to the state indicating “checked”.
In succession, the range Rb and the relative velocity V are arithmetically determined on the basis of the beat frequency components fbu[i] in the frequency increasing phase and the beat frequency components fbd[j] in the frequency decreasing phase in accordance with the expressions (3), (7), (8) and (10) mentioned hereinbefore.
Subsequently, unless the conditions that the range Rb is of positive or plus polarity and that the relative velocity V is “0” km/h are satisfied, the check/uncheck decision processing for the beat frequency components fbd[j] in the frequency decreasing phase is resumed.
On the other hand, when the range Rb is of plus polarity and when the relative velocity V is “0” km/h, the range Rg is computed on the basis of the range gate of interest in accordance with the expression (2) to be stored in an array of computation results of the range correcting values as an N-th computed range correcting value k[N].
Thereafter, the number N of times the range correcting value has been computed is incremented by “1”, and N range correcting values k[i] (i=1, 2, . . . , N) are averaged to thereby calculate a smoothed range correcting value, whereon the check/uncheck decision processing for the beat frequency components fbd[j] in the frequency decreasing phase is resumed.
Incidentally, the number N of times the range correcting value is computed and the range correcting value k[i] are initialized in advance in the main control processing.
In the conventional on-vehicle radar system, the range correcting values k[i] are arithmetically determined on the basis of the ranges Rb and Rg, respectively. In this conjunction, it is noted that since the range Rb is determined from the target located within the range of the range gate width, there always exist variances of the range Rb within the range of the range gate width. Under the circumstances, so far as the variances within the range of the range gate width are uniform, the range correcting value can be arithmetically determined with a reasonably high accuracy. However, in the actual environments, the variances mentioned above are not always uniform. As a result of this, there may arise the possibility that the range correcting value k[i] contains an error which corresponds to the range gate width, giving rise to a problem.
Certainly, it can be conceived to enhance the accuracy of the range correcting value by setting narrow the pulse width (i.e., range gate). In that case, however, S/N ratio of the received signal (echo) is degraded as the pulse width becomes narrower, which results in that probability of the target detection becomes low, incurring another problem.
Further, the vehicle velocity information derived from the output of the vehicle velocity sensor is required for computing the range correcting value, which makes it difficult to reduce the manufacturing cost of the on-vehicle radar system, involving still another problem.